Until recently, the usual way of permanently capturing an x-ray image involved the use of a sheet of film. A cassette of material opaque to visible light but not to x-rays is loaded with film sensitive to x-radiation and then positioned close to the object. The placement is along the exit path of the x-rays from the object and normal to a central axis along that path. When the source of x-ray radiation is activated an “x-ray shadow” is created as an exposed image on the film. The exposure is obtained without any intervening lenses. After all suitable preparations and precautions have been taken, including advance determination of the intensity and length of the exposure, the x-ray source is activated, and the film removed and developed. The film negative can then be viewed and interpreted by placing it against a light table.
The use of film as a medium for registering the x-ray image has both advantages and disadvantages. Advantages include simplicity and the ability to make the sheet of film as large as needed (within reason, of course), and that the sheet of film can be shaped to conform to a non-planar surface (e.g. a surface of a welded pipe joint). Disadvantages include the cost of the film and the time and cost of materials needed to develop the film. Since x-ray exposure times are generally longer than those of optical photography, there is always the chance that the image will be blurred by unwanted movement by the subject and that a reshoot will be required. Environmental awareness has also imposed costs: developing the film creates a silver bearing solution that is considered a pollutant if improperly disposed of, and can result in hefty fines.
In certain applications, including those in veterinary and some industrial applications, as well as in many medical applications, there is a trend toward using digital camera techniques to capture the image. In place of creating a large array of sensors responsive to x radiation and directly replacing the film with such an array (some films are as large as 14 by 17 inches) a scintillation screen in an otherwise light-tight environment is placed where the film would ordinarily be. A digital camera assisted by a lens system and having a much smaller array of sensors responsive to visible light then records the visible light image created by the x-rays impinging on the scintillation screen.
In veterinary, as in some medical and industrial settings, taking an x-ray involves placing the subject on a horizontal table where it can be positioned for the desired view, with a minimum of restraint and with the least likelihood of unwanted movement. Since an injured dog or cat may not appreciate having its hind quarters (or other parts) ‘squashed flat’ for the time needed to set up and then take the x-ray image, animals are often first sedated, and placing them on a horizontal table is often an excellent way to enlist the help of gravity and ensure that they ‘stay put.’
The surface of the table is one that is strong enough to hold the subject, can withstand frequent cleaning, is not opaque to x-ray radiation, but is opaque to light of optically visible wavelengths. A suitable source of x-ray radiation is movably mounted above the table, and it is common for a projected image of visible light, perhaps involving a cross-hair or other reticule shape, to allow the technician to best position the pattern of uniform x-ray irradiation onto the desired portion of the subject. Just beneath the table, and within a light-tight environment, is a scintillation screen that will fluoresce to produce a scintillation image according to the varying amounts of x-ray radiation reaching each different location of the scintillation screen. If Fido the dog has a broken bone, it is at this point that it is (or would be, if we implemented the means to do so . . . ) first be visible by inspection of the scintillation image. (So far we have described what is essentially a fluoroscope.) However, to make the scintillation image permanent for subsequent inspection, analysis and future reference, it needs to be captured by a camera sensitive to visible light.
To continue with a particular type of veterinary application, which may be taken as illustrative of the state of affairs, the table of the previous paragraph becomes the top surface (or perhaps another part) of a light-tight cabinet or other enclosure. To capture the scintillation image, a digital camera, which may be of a conventional sort found in commerce, is aimed along an optical axis normal to the scintillation screen and that intersects the anticipated center of the scintillation image. One or more mirrors may be employed in the visual optical path to allow the table to remain at a convenient height. It has been found quite feasible to replace a 14″×17″ film cassette with a 14″×17″ scintillation screen and photograph that with a digital SLR (Single Lens Reflex) camera whose image sensor is of the 24×36 mm size used to emulate the older traditional negative size of the venerable “35 mm (film) camera” (whether SLR or otherwise). The two aspect ratios are not identical (0.82 for 14×17 and 0.67 for 24×36), so that some minor cropping might occur, but this mis-match in aspect ratio is generally not noticeable, or if it is, will be neither fatal nor objectionable. Cameras having ten megapixels or more have been employed with generally satisfactory results.
This has been an agreeable solution for “large/medium-sized animals” in a veterinary setting where a traditional film cassette of 14″×17″ would otherwise be in order, as well as for some human medical applications, such as an x-ray image of an adult's chest. Part of what is meant by “agreeable” in the preceding sentence is that the camera lens settings and x-ray exposure times are practical (i.e., relatively short, lest Fido move whilst the image is being shot, or, at the other extreme, be overdosed with either high intensity or long duration x-radiation), and also that the resulting resolution and other properties of the viewable image are satisfactory. Another part of what is meant by “agreeable” is suitable dynamic range: that the exposure avoids both saturation of, and failure to excite, fluorescence in the scintillation screen, and also correspondingly, that the visible fluorescence neither saturates nor fails to excite the optical sensors in the camera. However, to obtain these agreeable outcomes for large/medium-sized animals the distance from the camera lens to the scintillation screen becomes essentially fixed.
Now suppose that the subject is a “small animal” in a veterinary setting, or in a human medical setting, the imaging of “small” body parts such as hands or feet. Say, that if it were a film set-up, a sheet of, say, 6″×6″ film would be used in a suitable cassette (or perhaps 6″×8″, etc., as the choice of 6″×6″ herein is merely illustrative). Furthermore, we are quite desirous that the x-ray exposure itself (intensity, length of time) conform to accepted standards, and that it not amount to a larger than usual dosage of x-radiation. Unfortunately, certain departures from “agreeable” (problems) attend using the conventional digital camera arrangement described in the preceding two paragraphs. In particular, certain practical circumstances exists that, in conjunction with the strong urge to keep the x-ray exposure/dosage unchanged, precludes moving the camera in closer to the scintillation screen to maintain (essentially) the same number of exposed pixels over the size of the (now smaller) actual visible scintillation image. That is, for a 6″×6″ area within the complete 14″×17″ area, about ⅚th of the total area (or about ⅘th of the total area for 6″×8″) is ‘empty’ of any useful information! Accordingly, then, without moving the camera to change the length of the optical path, only about ⅙th (or only about ⅕th for 6″×8″) of the sensors in the camera will “see” what is the desired image: we would be losing potential camera resolution, in that a camera pixel now corresponds to a much larger percentage of the smaller subject. An opportunity for a very desirable potential increase in pixel resolution with which to view and interpret the x-ray image of some pussy cat's hind-quarters has been missed. (Put another way, the same hind-quarters shot of a large dog might be thought to have a good quality of resolution, while the same shot for the cat seems wanting. Roughly speaking, a feature of interest in the dog's radiograph has five times the number of camera pixels as the corresponding feature in the radiograph for the cat.) We shall say that, for the cat, “full” camera resolution has not been obtained. We should like to maintain “full” camera resolution for smaller subjects (that is, have the smaller scintillation image of interest fill, or nearly fill, the entire array of the camera's visual imaging sensors). The contrary notion that we should sacrifice image detail simply because the subject is small seems to us to be very undesirable: equal rights for cats and small rodents (as well as for hands and feet)!
Furthermore, the large percentage of unexposed pixel sensors in the camera's array of optical sensors may disturb the camera's idea of what constitutes a proper optical (visual light) exposure, leading to overexposure of the information that is present. (Suitable exposure for an image having a large dynamic range of intensity is recognized problem for digital cameras, and there are many conventional techniques to deal with this, such as monitoring predefined zones of interest and various types of averaging. All we are suggesting here is that automated exposure monitoring solutions will work better when the image tends to fill the entire array of optical sensors, and that the 6″×6″ out of 14″×17″ scenario is a genuine one that also complicates even the ‘smartest’ camera's automatic exposure monitoring task, if such were in use.)
If we could move the camera closer there would still be (on average and for a 6″×6″ subject image) only about ⅙th the light available for imaging when compared to that available from a fully illuminated 14″×17″ image (think in terms of uw/cm2 for light originating at the scintillation screen—the larger scintillation screen produces a larger total amount of light!) However, and as is well known, the uniform omnidirectional nature of the light rays precludes the need to perform an adjustment to the aperture of the camera's lens, although we will most likely need to re-focus the lens. Now, even if the bulk of the camera is, for understandable convenience, outside the light tight environment, the lens assembly itself might not be, the better to prevent damage and other accidental mischief. In and of itself, this lack of easy physical access to the lens need not be a problem nor an inconvenience. On the one hand, for many cameras control over these functions resides at a menu driven user interface rather than on a user applied twisting motion to some knurled external ring on the lens assembly proper. On the other, we shall see that such internal automation of the camera's lens assembly is not required.
So, now we are all set to move the camera in along the optical path, and are prepared even to re-focus it at the new distance. Unfortunately, we have to move it quite a ways (to about 40% of what it was to go from 14″×17″ to 6″×6″). Assuming that there is some transport mechanism to actually translate the camera along the optical path, we no more than begin to move it than we run into a rather formidable obstacle: the camera needs to pass through (or even ultimately occupy the position of) one or more of the mirrors used to fold the optical path and keep the table at a convenient height. A zoom lens comes to mind, but upon reflection we appreciate that its use requires alteration of the x-ray exposure to accommodate a longer optical exposure, and is therefor not a desirable solution. It also represents a significant expense, as well as an additional operational complexity that can contribute to error.
Still, we lust after the shorter optical path for “smaller” subjects, and its promise of maintaining “full” camera resolution in photographing the final scintillation image. What to do?